# Ligand exchange reactions… are they one way or reversible?

Are ligand exchange reactions one-way reactions or reversible? I know this is a very silly question but its not said outright in any place...

For example, in my high school chemistry book, these two ligand exchange reactions are shown:

$$\ce{[Cu(H2O)6]^{2+} (aq) + 4Cl- (aq) <=> [CuCl_4]^{2-} (aq) + 6H2O (l)}$$

$$\ce{[Cu(H2O)6]^{2+} (aq) + 4NH3 (aq)-> [Cu(NH3)4(H2O)2]^{2+} (aq) + 4H2O (l)}$$

The first is reversible but the second is one-way

Also, my teacher stated that ligand exchange reactions are irreversible... this confused me further

Can someone please tell me what is right...

## 1 Answer

Let's take the binding of $\ce{O2}$ and $\ce{CO}$ to hemoglobin as example. $\ce{CO}$ does bind much better than $\ce{O2}$!

We can consider the reaction of oxygen-loaded hemoglobin with $\ce{CO}$ a ligand exchange reaction. If that would be completely irreversible, the logical consequences in the case of an intoxication would be: [...] six feet under ;)

Instead, intoxicated patients are treated with oxygen at higher pressure.

To me, that sounds like Le Chatelier in the ICU and it only makes sense in the case of equilibria.

• so ligand exchange reactions involving transition metal complexes are actually all reversible...? – Eliza Jan 27 '14 at 15:19
• @Eliza Yes, in principle. But the affinity of some metals to particular (polydentate) ligands can be very strong. So you have a bad time trying to replace these ligands with some others. – Klaus-Dieter Warzecha Jan 27 '14 at 15:30
• in that case, would it still be a reversible reaction but the equilibrium would almost be to the right (i.e. the side of the reaction with the complex ion with polydentate ligand)...? – Eliza Jan 27 '14 at 17:09
• From a theoretical point of view, I'd still say that it's reversible. Practically, cleavage (and re-building) of the complexes might be achieved by playing with the pH. Under acidic conditions, nitrogen atoms in the ligand would be protonated and the ligand thus cannot "bite". – Klaus-Dieter Warzecha Jan 27 '14 at 17:42